The present invention relates to a method of and a system for coupling a grounded color changer and associated grounded supplies of conductive paint to electrostatic spraying equipment, while maintaining electrical isolation between a high voltage at the spraying equipment and the color changer and paint supplies.
Color changers for spray coating apparatus have application in industrial operations where articles are to be spray coated at a station or as they move along a production line. Where the articles are to be coated a wide variety of colors, it generally is not practical to establish separate spray stations or production lines for each color, or even to spray a long sequence of articles one color, then another long sequence a second color, etc. Instead, it is desirable to make color changes rapidly and simply at a single station.
Electrostatic spray coating devices have an increased painting efficiency over non-electrostatic types. When painting with an electrostatic spraying apparatus, it is necessary to have some means for applying a charge to the paint. In some apparatus, charging is accomplished by an electrode connected to a high voltage supply and placed in close proximity to or in contact with the paint either just prior or close to its point of atomization. In rotary atomization apparatus, the rotary atomizer is ordinarily made of a conductive material and connected to the power supply, and itself is the electrode. Whichever type of apparatus is used, the charging potential is usually on the order of several tens of kilovolts, and the electrostatic charging process works well when spraying nonconductive paints. However, when spraying conductive paints, such as waterborne or water based paints, precautions must be taken to prevent the high voltage at the spraying apparatus from shorting to ground through a conductive column of paint being delivered to the spraying apparatus.
One known approach to prevent shorting the high voltage to ground when spraying conductive paints is to isolate the entire paint supply and color change system from ground potential. This allows the paint supply and color change system to "float" at the charging potential, but has the drawback that a large amount of electrical energy is capacitively stored in the system. To prevent this energy from presenting a shock hazard to operating personnel, it is necessary to provide a protective enclosure around the color changer and paint supplies, which increases costs and requires that the spraying operation be shut down and the system electrically discharged whenever it is necessary to replenish the supplies of paint. Also, during operation of the system, the large amount of capacitively stored energy increases the potential for injury to an operator.
More recently, so-called isolation systems or voltage blocks have been employed to couple a grounded color changer and associated grounded supplies of conductive paint to an electrostatic sprayer in order to isolate the color changer and paint supplies from the high voltage at the sprayer. Such voltage block systems often have at least one reservoir into which a volume of one color of paint to be sprayed is introduced. After the reservoir is filled with paint, at least part of the fluid line between the reservoir and color changer is flushed clean to electrically isolate the paint in the reservoir from the color changer and paint supplies. The paint in the reservoir is then delivered to the sprayer for being emitted in an electrostatically charged atomized spray, with the cleaned portion of the fluid line between the reservoir and color changer then providing electrical isolation to prevent the high voltage at the sprayer from being coupled to the grounded color changer through a conductive paint path extending between the sprayer and the color changer. At the end of spraying the one color of paint, the high voltage is removed from the sprayer and the reservoir, fluid lines and sprayer are cleaned of paint of the one color and reloaded with paint of another color. Often such voltage block systems have two reservoirs which alternately receive and deliver selected colors of paint to the sprayer, so that one reservoir may be cleaned and reloaded with paint while the other reservoir delivers paint to the sprayer, thereby to shorten color change times. One such dual reservoir voltage block system is taught by U.S. Pat. No. 4,932,589, assigned to the assignee of the present invention. Other voltage block systems are taught by U.S. Pat. Nos. 4,771,729, 4,792,092 and 4,962,724.
Although rapid color changes can be made with voltage block systems that utilize two reservoirs, the art does not provide particularly fast color changes when only a single reservoir voltage block is used. This is because of the number and the nature of the steps that are performed with known single reservoir voltage block systems in order to clean the systems to make a color change.